This invention relates to the mechanical connection of modular structural elements, such as those of a positioning apparatus or space frame. More particularly, the invention relates to the design of disconnectable couplings functioning as a means to this end.
In the context of designing a positioning apparatus, the need arises to design structural elements of same that are rigid, therefore permitting accuracy of placement. Such an apparatus must also transmit mechanical power in order to execute placement, e.g. as in the case of an articulated robot arm. Positioning per se implies using an apparatus to move an object in a given linear direction and/or rotate it. Thus, an active structural element of a positioning apparatus must be able to transmit linear force as well as rotational force, commonly called torque.
Further, in the context of designing a positioning apparatus that can be readily assembled from or disassembled into a set of modular components, the coupling design used in connecting these components to each other must reliably transmit torque as well as linear force, with mechanical precision. Here, this kind of coupling will be called a “Torque Coupling”, not to be confused with a conventional power coupling or clutch.
To maintain positional accuracy, a Torque Coupling must be capable of being accurately and repeatably assembled, and it must be capable of transmitting linear force and/or torque without mechanical slippage, either linear or rotational. These requirements distinguish the design of a Torque Coupling from that of a conventional power coupling or clutch, such as used in the drive shaft of a motor. In fact, for reasons of practicality the latter are deliberately designed to accommodate misalignment, rendering them unsuitable for true kinematic applications.
As it happens, a parallel context exists for which a Torque Coupling is suitable, that of a collapsible Space Frame, i.e. a space frame which can readily be assembled from or disassembled into modular elements. Here, the term “Space Frame” means a rigid assembly of predominantly linear struts, connected to each other at their ends. Rigidity is obtained by the organization of the assembly into a two or three dimensional system of triangles, since the triangle is the only fundamentally rigid shape that can be formed by linear struts. These triangles need not be regular.
By their nature, Space Frames are both rigid and lightweight, making them suitable for applications requiring both physical stability and economy of material (i.e. mass), such as bicycle frames, telescope mirror supports, space stations, booms of construction cranes, truss-type bridges, and various types of scientific apparatus.
Because of the intrinsic geometry of a Space Frame, its component struts can only be subjected to linear (tensile/compressive) or rotational (torsional) forces—but not bending—as the entire assembly is subject to a variety of loads. Therefore, the fundamental requirements of a Torque Coupling connecting strut components in a Space Frame are the same as for a positioning apparatus, i.e., the Torque Coupling must be capable of being accurately and repeatably assembled, and must reliably transmit linear and/or rotational force without mechanical slippage.
Previously, designs such as the Bicycle Torque Coupling (BTC) of Smilanick, U.S. Pat. No. 5,431,507, utilized a pair of cylindrical fittings bonded to mating strut ends. These fittings each had a set of teeth that interlocked with each other as the two fitting halves were clamped together. This interlocking prevented mechanical slippage due to rotational force. The two fittings were clamped together by a captive threaded casing, or “nut”, that slid over one fitting and screwed onto external threads on the other fitting. This nut contained external notches used for tightening with a specialty wrench. The BTC is produced commercially by the S and S Machine Co. of Roseville, Calif., and is used to attach collapsible bicycle frames together.
The BTC-type design presents several problems in certain applications involving a positioning apparatus. First, it is possible to assemble the two fittings together such that the teeth actually touch at their tips instead of interdigitating. In this position it is also possible to engage a few threads of the nut and hold the assembly together, giving the impression that the assembly has been correctly assembled, even though it's actually at risk of failing in torsion. Also, this mis-assembly will cause the positioning apparatus as a whole to be mis-aligned. As mentioned, it cannot be readily determined by visual inspection whether a BTC-type coupling has been incorrectly assembled in this manner. Therefore it is problematic to validate an assembly with many couplings, e.g. a field application. Therefore the BTC and similar designs are unreliable in fail-safe applications, i.e. applications in which failure would be a high-consequence event.
Second, an external wrench is required to assemble a BTC-type coupling. This tool can be misplaced or lost, rendering emergency assembly or disassembly problematic. Worse, in an installation where loose items can cause catastrophic damage by falling into sensitive equipment, an external wrench requires tethering, which can complicate assembly. Furthermore, an external wrench may be difficult to use in environments where the apparatus must be assembled in difficult or hostile environments requiring a technician to wear gloves. Also, an external wrench in the hands of an assembly technician yields unpredictable results in terms of the tightened condition of a BTC-type nut unless a torque wrench is used, but the latter would be quite cumbersome and would require the concurrent use of a dedicated workstation to hold and secure the coupling during tightening.
Third, even when correctly assembled, it is not possible to determine by visual inspection whether a BTC-type nut has been properly tightened or is in fact loose, a condition that also renders failsafe operation problematic.
Fourth, the use of a threaded nut in a BTC-type design represents, ultimately, a reliance on the frictional forces induced in the threads of the coupling as they are twisted to elastically comply with each other. This is actually a form of clamping, in which the nut functions as a spring, of sorts, due to its elastic character. The reliance on friction, springs or clamping as an attachment scheme is fundamentally unreliable in environments requiring immersion in highly viscous (and therefore lubricating) fluids, such as encountered in a certain class of underground neutrino detectors. Also, friction-based attachments schemes are undermined by vibration, regular or intermittent, e.g. shocks induced by dropping, etc. In industry, nuts installed in mechanically hostile environments typically have some kind of a secondary, positive, mechanical lock to prevent them from backing out. However, these extra parts add complexity, and therefore increase installation difficulty, including the risk of lost parts, or of parts dropped into peripheral instrumentation, possibly resulting in catastrophic damage.
Fifth, the BTC-type threaded nuts are bulky in order to incorporate a set of female threads, as well as to have sufficient structural integrity to withstand assembly wrenching forces. This bulk increases the overall weight of an assembly. It also increases the maximum cross-section of a strut assembly, reducing clearances through any bulkhead.
Sixth, assembly of a threaded BTC-type nut assembly is time-consuming, because the thread engagement must be carefully started and then turned several times, a wrench must be used, and the wrench force must be gauged somehow, possibly requiring the use of a dedicated assembly workstation and torque wrench.
Accordingly, several objects and advantages of the invention described herein, named the Quick Torque Coupling (QTC), are:
First, the QTC eliminates vague and indeterminate conditions of intermediate assembly, such that the assembled condition falls simply into two distinct categories—it is either locked or unlocked. This eliminates the danger of a faulty assembled condition that might fail and/or cause the parent positioning apparatus to be misaligned. Therefore, assembly of the invention is highly reliable and suitable for fail-safe applications, which is an object and advantage.
Second, no wrenches, tools or other loose hardware are required for the primary assembly of the QTC. The QTC is self-contained, so there is no risk of not having a tool available for emergency assembly or disassembly. Also, there is no need for a specialized assembly workstation with tethered tools, and there is no danger of dropping either tools or secondary locking hardware into nearby equipment or instrumentation. Therefore, assembly of the QTC is simple and self-contained, which is an object and advantage.
Third, the physical engagement of the QTC assembly can be readily inspected by eye, and a “go”/“no-go” status of each coupling can be quickly and easily assessed. Therefore the QTC can be simply and easily tested, which is an object and advantage.
Fourth, the QTC locks by direct positive engagement of triply interlocking teeth, eliminating the use of any kind of primary clamping, spring-loaded, or friction based mechanism. Thus, the integrity of the assembled condition of the QTC is fundamentally fail-safe. There is no virtually no danger of unexpected self-disassembly due to the undermining of a threaded coupling by either lubrication and/or vibration or shock. Therefore, the mechanism of the QTC is simple and reliable, which is an object and advantage.
Fifth, the QTC eliminates the need for a bulky and heavy captive nut, substituting a low-profile light-weight locking collar. The QTC is therefore relatively compact and light-weight compared to the prior art, such as the BTC, above, which is an object and advantage.
Sixth, because the QTC is straightforward, simple, and requires no tools, it is therefore relatively rapid to assemble, compared to the prior art. This reduces installation and maintenance time, which is an object and advantage.
Further objects and advantages of the QTC will become apparent from a consideration of the ensuing description and drawings.